I AM IN THE MARKET FOR A PRE-BAN AR-15 CARBINE. I HAVE FOUND A SITE, WWW.PREBANARMS.COM, WHICH STOCKS A GOOD SUPPLY OF AR-15 CARBINES. THEY HAVE AR-15 A2's LISTED FOR $2,800 OR SO, AND AR-15 SP-1's FOR AROUND $1,000 LESS. AM I MISSING SOMETHING HERE? BOTH ARE DESCRIBED AS BEING "UNBLOCKED". ALSO, IS THE R6001 AS GOOD AS THE R6520? NEED HELP.

So whaddya wanna know?
AR15A2s are more expensive because they were not in production for as many years as the SP1. They of course have the faster twist for heavier bullets and the heavier barrel, at least from the front sight forward. They also have the A2 sights and the forward assist.
If you like a lighter rifle and shoot 55 grain bullets, get the SP1 and save money. If you shoot heavier bullets and want the extra adjustable features of the A2 sights, then buy the A2. I'd take the SP myself.
"Unblocked" refers to the block of metal in the lower receiver Colt began installing (to kiss the ass of their government masters, not because they were required to). This block fills in the spot where the auto sear would go on a selective fire rifle allegedly making it harder to convert it.
I can never remember what the dif is between all the model numbers . . .

IMO the A2 is better. It was improved over the SP1 as noted: handguard, forward assist, sights,etc...
The sp1 is more nostalgic if that is worth anything.
Note: those prices seem very high, I would shop around. I bought the colt ar15-a2 20", model 6600 (green box) for $1400 new. The blue label 6601 goes for $1000-$1200.
Not sure what the SP1 sells for, probably around the same.

the best of the bunch is the r6500. esentially a ar15a2 with all the upgrades, but with field sights and brass deflector. this model in military form became the canadian c7. to me this is the best iron sighted ar ever made for overall use. the sp1 is a great gun too. your call and pref.
steve